1. Field of the Invention
This invention pertains to optical fibers for discharging laser energy laterally to an axis of the optical fiber. More particularly, this invention pertains to such an optical fiber and a method for making the same.
2. Description of the Prior Art
So called “side-firing” optical fibers discharge light laterally or transverse to a longitudinal axis of the optical fiber as opposed to discharging light from a laser tip in a direction substantially parallel to the axis of the optical fiber. An example of a side-firing optical fiber is shown in U.S. Pat. No. 4,785,815 to Cohen dated Nov. 22, 1988. Particularly, FIGS. 7 and 9 of the '815 patent show optical fiber tips for discharging energy laterally relative to the axis of an optical fiber.
Optical fibers are fragile when not protected by appropriate cladding, jacket and buffers. Currently, the construction of a side-firing optical fiber requires removal of these components and addition of other materials, a process which can be difficult or expensive to manufacture in a manner which preserves the desired optical qualities while avoiding damage to a fragile optical fiber during the assembly process. A more simple construction of a side-firing optical fiber is disclosed in U.S. Pat. No. 5,537,499 to Brekke, dated Jul. 16, 1996. As shown in FIGS. 7-11 of the '499 patent, an optical fiber is placed within a tubular member formed of silica. The optical fiber has an inclined end surface within a gas filled chamber to cause reflection of light traveling along the axis of the optical fiber to exit the optical fiber tip transverse to the optical fiber axis. The optical fiber tip is fused to the silica of the tubular member to create a continuous material from the optical fiber tip through the silica tubular member to avoid alteration in an index of refraction throughout the light path.
While the design of the '499 patent is an efficient design for many applications, it has limitations. Specifically, the design of the '499 patent is limited to a optical fiber having a cladding which can withstand the thermal energies required during the process of fusing the optical fiber tip to the silica tubular member. The fusion process results in a melting of the optical fiber in the silica tubular member to form a continuous material. This occurs at the melting point of fused silica, a temperature of about 1470° C. If the cladding of the optical fiber cannot withstand such temperatures, the cladding will melt resulting in at least a portion of the length of the optical fiber being unclad and thereby not reflective to incident internal energy. In the '499 patent, such cladding is a so-called “doped fused silica cladding” which can withstand the temperatures of the welding process of the optical fiber tip to the silica tubular member.
Optical fibers having doped fused silica cladding are acceptable for many applications. For most optical fibers, the doped fused silica layer is approximately 20 microns thick. There is only a small index of refraction difference between the fused silica core of the optical fiber and the doped fused silica cladding. The critical angle of an optical fiber is determined by the index of refraction difference between its core and cladding. The critical angle is defined as the maximum incidence angle from the centerline of an optical fiber for total internal reflection. The smaller the index of refraction difference between the core and cladding, the more collinear the laser light must be when entering the optical fiber. For most commercially available optical fibers using a fused silica core and a doped fused silica cladding, the critical angle of the optical fiber must be less than 13 degrees. A critical angle of less than 13 degrees corresponds to a numerical aperture of 0.22 (which is approximately the arcsine of the critical angle). Many commercially available flash lamp lasers have very small divergence angles are ideally suited for use with the design of the '499 patent having doped silica cladding on a silica core optical fiber.
In addition to so-called flash lamp lasers, diode lasers are becoming increasingly popular due to their lower cost, smaller physical size and greater reliability. However, diode lasers are significantly constrained as to power output, minimum spot size and divergence angle. As a result, applications using diode lasers need optical fibers for delivering the laser energy which maintain high optical efficiency to provide adequate power to the optical fiber tip and accept a divergent beam significantly greater than commercially available side firing optical fibers which use optically efficient designs such as the '499 patent.
Commonly, the divergence angle of most diode lasers is approximately 22 degrees which requires an optical fiber with a numerical aperture of 0.37 to capture and retain all incident energy. This is significantly greater than the maximum tolerable numerical aperture of commercially available fibers which use a design such as that of the '499 patent containing a silica core optical fiber with a doped fused silica cladding. Accordingly, the use of such a diode laser with such a design results in a substantial loss of power during transmission of the laser energy along the optical fiber because the incidence angle of the laser is larger than the critical angle of the optical fiber.
A higher numerical aperture would be possible with the design of the '499 patent if the doped silica cladding were to be replaced with any one of a number of different commercially available plastic claddings having a higher index of refraction difference between the cladding and the silica core of the optical fiber. Unfortunately, such plastic claddings have melting temperatures significantly lower than that of the silica core. As a result, the fusion process described in the '499 patent cannot be used with such optical fibers since, during the fusion process, a substantial length of the plastic cladding will melt leaving a substantial length of the optical fiber core unclad. This substantial length results in loss of laser energy. Since laser diodes already operate at relatively low power outputs, such a loss of energy is unacceptable for most applications.
It is an object of the present invention to provide an optical fiber having the advantages of the '499 patent while avoiding the aforementioned disadvantages.